Future U.S. government standards will probably require the reduction of nitrogen oxide gases present in the exhaust from an automobile to levels not exceeding about 100 p.p.m. Also, most carbon monoxide and hydrocarbons present in the exhaust will have to be eliminated. A contemplated system for treating these exhaust contaminants includes a two-stage catalytic converter. The first stage or reactor treats exhaust gases directly coming from the exhaust system under reducing conditions. In this first reactor, the nitrogen oxide gases in the exhaust react with hydrocarbons and carbon monoxide in the exhaust. Just prior to or in the second stage or reactor, oxygen is injected into the exhaust stream. Oxidation occurs in this second stage, where the hydrocarbons and carbon monoxide react with the oxygen. Such a two-stage catalytic converter is being designed, with each stage containing different types of catalyst. Such a converter requires that the internal combustion engine be operated at fuel-rich conditions, insuring the presence of sufficient carbon monoxide and hydrocarbons to react with the nitrogen oxide gases.
The catalyst in the first stage is called the NO.sub.x catalyst. It promotes the reduction of nitrogen oxides. Several different and sometimes competing reactions can and do occur as the exhaust flows over the NO.sub.x catalyst. The most important reactions are: EQU NO.sub.x + CO.fwdarw. N.sub.2 + CO.sub.2 ( 1) EQU no.sub.x + H.sub.2 .fwdarw.NH.sub.3 + N.sub.2 + H.sub.2 O (2) EQU 2co + o.sub.2 .fwdarw.2co.sub.2 ( 3) EQU 2h.sub.2 + o.sub.2 .fwdarw.2h.sub.2 o (4)
reactions Nos. 1 and 2 are the reactions responsible for eliminating NO.sub.x as an exhaust pollutant. Advantageously, the NO.sub.x catalyst favors these reactions rather than Reactions Nos. 3 and 4. Preferably, the NO.sub.x catalyst favors Reaction No. 1 over a wide range of temperatures, for example, from 700.degree. to 1,700.degree. F. Reaction No. 2 is undesirable because ammonia formed in the first stage of the converter will be converted to nitrogen oxide in the second stage. This ammonia formation will defeat, at least in part, the purpose of the converter. Consequently, a desirable NO.sub.x catalyst will not promote Reaction No. 2 or will minimize formation of NH.sub.3 in favor of N.sub.2. Reduction to NH.sub.3 occurs most frequently at low temperatures. Reactions Nos. 3 and 4 are undesirable because carbon monoxide and hydrogen are oxidized. The NO.sub.x catalyst requires that some carbon monoxide be present in the exhaust. However, there is also present in the exhaust some oxygen, which reacts with carbon monoxide, using the carbon monoxide least advantageously. The preferred NO.sub.x catalyst will selectively promote the reaction of carbon monoxide with NO.sub.x in the presence of small amounts of oxygen (up to about 1%). There has now been developed an improved NO.sub.x catalyst.